Laser wavelength control unit with piezoelectric driver

ABSTRACT

An electric discharge laser with fast wavelength correction. Fast wavelength correction equipment includes at least one piezoelectric drive and a fast wavelength measurement system and fast feedback response times. In a preferred embodiment, equipment is provided to control wavelength on a slow time frame of several milliseconds, on a intermediate time from of about one to three millisecond and on a very fast time frame of a few microseconds. Techniques include a combination of a relatively slow stepper motor and a very fast piezoelectric driver for tuning the laser wavelength using a tuning mirror. A preferred control technique is described (utilizing a very fast wavelength monitor) to provide the slow and intermediate wavelength control and a piezoelectric load cell in combination with the piezoelectric driver to provide the very fast (few microseconds) wavelength control.

FIELD OF THE INVENTION

[0001] This application is a continuation-in-part of Ser. No. 09/501,160filed Feb. 9, 2000, Ser. No. 09/597,812, filed Jun. 19, 2000 and Ser.No. 09/684,629, filed Oct. 6, 2000. This invention relates to lasersand, in particular, to correcting wavelength shift in lasers.

BACKGROUND OF THE INVENTION Light Sources For Integrated CircuitFabrication

[0002] An important use of gas discharge excimer lasers is to providehigh quality light sources for integrated circuit fabrication. Theselight sources are used by stepper machines and scanner machines forselectively exposing photoresist in a semiconductor wafer fabricationprocess. In such fabrication processes, the optics in the stepper andscanner machines are designed for a particular wavelength of the laserbeam. The laser wavelength may drift over time and, thus, a feedbacknetwork is typically employed to detect the wavelength of the laser andto control the wavelength within a desired range.

Prior Art Wavelengths Controls

[0003] In one type of feedback network used to detect and adjust thewavelength of a laser, a grating and an etalon each receive a smallportion of the emitted light from the laser. The position of a band oflight reflected from the grating determines the wavelength of the laserbeam to a coarse degree. The etalon creates an interference patternhaving concentric bands of dark and light levels due to destructive andconstructive interference by the laser light. The concentric bandssurround a center bright portion. The diameter of one of the concentricbands is used to determine the wavelength of the laser to a fine degree,such as to within 0.01-0.03 pm.

[0004] Various feedback methods are well known for wavelength tuning oflasers using the measured wavelength values. Typically the tuning takesplace in a device referred to as a line narrowing package (LNP) or linenarrowing module. A typical technique used for line narrowing and tuningof excimer lasers is to provide a window at the back of the laser'sdischarge cavity through which a portion of the laser beam passes intothe LNP. There, the portion of the beam is expanded with a prism beamexpander and directed to a grating which reflects a narrow selectedportion of the laser's broader spectrum back into the discharge chamberwhere it is amplified. The laser is typically tuned by changing theangle at which the beam illuminates the grating. This may be done byadjusting the position of the grating or providing a mirror adjustmentin the beam path. These prior art wavelength control techniques are veryeffective in maintaining average wavelength values within a desiredrange over relatively long time periods. However, they have not beenvery effective in controlling wavelengths over short time periods ofabout 3 to 30 milliseconds or very short time periods of about 1-3milliseconds or less. In prior art wavelength feedback controltechniques had response times of a few milliseconds which was the timerequired to detect a shift in the wavelength and adjust the illuminationangle.

Wavelength Chirp

[0005]FIG. 1 is a graph 10 which illustrates the wavelength shift duringa burst of pulses from a laser operating at 1000 Hz. In particular, FIG.1 indicates a wavelength shift of about +0.1 pm to about −0.09 pm from adesired wavelength output over a time period of about 35 milliseconds.Wavelength shifts of this type are referred to as wavelength “chirp”.These chirps often are very predictable, coming at the same time duringeach of many bursts of pulses. As shown in FIG. 1, after the wavelengthchirp, the wavelength output settles down to wavelength shifts occurringrapidly and randomly but with a maximum magnitude of less than about0.05 pm. Applicants believe that this wavelength chirp near the start ofa burst of pulses is primarily due to changing acoustic thermaldisturbances within the discharge region of the laser. These shiftsoccur over time periods in the range of a few to several milliseconds.Conventional wavelength correction techniques do not adequately correctthese large and sudden wavelength shifts near the beginning of eachpulse.

Vibration

[0006] Conventional prior art wavelength correction techniques also arenot adequate to correct the small very rapidly occurring (highfrequency) wavelength shifts that occur throughout the burst. The highfrequency wavelength shifts are believed to be caused primarily byvibrations of the laser optical components including those in the LNPitself. Most of the vibration type shifts are believed to be primarilyattributable to laser's rotating fan and its motor drive and to theperiodic electric discharges of the laser. Vibration modes may beamplified by resonant conditions of various laser structures includingthe LNP and its components.

Energy Chirp

[0007] Excimer lasers operating in a burst mode also produce a pulseenergy chirp similar to the wavelength chirp. Prior art methods havebeen disclosed to minimize the pulse energy chirp. One such method isdescribed in an article by the inventors' co-workers, “Advanced KryptonFluoride Excimer Laser for Microlithography, SPIE Vol. 1674,”Optical/Laser Microlithography V, (1992) 473-484, see page 480.

[0008] What is needed is equipment to control the wavelength of gasdischarge lasers over short and very short time periods in the range of1 to 3 milliseconds or less.

SUMMARY OF THE INVENTION

[0009] The present invention provides an electric discharge laser withfast wavelength correction. Fast wavelength correction equipmentincludes at least one piezoelectric drive and a fast wavelengthmeasurement system and fast feedback response times. In a preferredembodiment, equipment is provided to control wavelength on a slow timeframe of several milliseconds, on a intermediate time from of about oneto three millisecond and on a very fast time frame of a fewmicroseconds. Techniques include a combination of a relatively slowstepper motor and a very fast piezoelectric driver for tuning the laserwavelength using a tuning mirror. A preferred control technique isdescribed (utilizing a very fast wavelength monitor) to provide the slowand intermediate wavelength control with a piezoelectric driver and apiezoelectric load cell in combination with the piezoelectric driver toprovide the very fast (few microseconds) wavelength control.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a prior art graph of measurements of wavelength driftover a burst of pulses from a laser.

[0011] FIGS. 2A-D are prior art graphs of measurements of wavelengthdrift over four sequential bursts of pulses from a laser.

[0012]FIG. 3 is a graph of measurements of wavelength drift over a burstof pulses from a laser that has its wavelength output corrected using aslow response stepper motor.

[0013]FIGS. 4, 4A and 4B show a proposed technique for providing fastand finer wavelength control.

[0014]FIG. 5 is a drawing of a wavemeter.

[0015]FIGS. 5A and 5B show how wavelength is calculated.

[0016]FIG. 6 is a drawing depicting the surface of a photo diode array.

[0017]FIG. 6A shows how grating and etalon images appear on the surfaceof the FIG. 6 photo diode array.

[0018]FIG. 7 shows the arrangement of wavelength calculating hardware.

[0019]FIG. 8 and 8A show a fast mirror driver and a control module.

[0020]FIG. 9 shows a feedback control algorithm flow chart.

[0021]FIG. 10 shows test results.

[0022]FIG. 11 shows a feedforward control algorithm flow chart.

[0023]FIG. 12 shows test results.

[0024]FIGS. 13A and 13B show test results.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Wavelength Shifts

[0025]FIGS. 2A, 2B, 2C and 2D are graphs 16, 18, 20 and 22,respectively, which illustrate the wavelength shifts over foursequential bursts of pulses from a laser. Graphs 16, 18, 20 and 22reveal that the shape or pattern of the wavelength drift of thewavelength chirp from a particular laser is similar from burst-to-burst.Data averaged over 60 pulses is shown as the solid lines in graphs 16,18, 20 and 22. These data demonstrate a substantial relativelypredictable wavelength chirp during the first 30 ms of the bursts andrelatively random smaller wavelength shifts throughout the bursts.

Chirp Correction with Prior Art Stepper Motor

[0026] The graph 26 of FIG. 3 illustrates the wavelength shift over aburst of pulses from a laser that has its wavelength output correctedusing a prior art stepper motor. A circled section 28 of graph 26reveals a significant reduction in the magnitude of wavelength driftduring the wavelength chirp period of the laser, relative to the circledsection 12 of graph 10 of FIG. 1. In particular, the magnitude of themaximum wavelength shift during the wavelength chirp period shown ingraph 26 is about 0.05 pm.

[0027] Stepper motor control line 29 indicates that a stepper motor,which controls the tuning mirror positions for tuning the laser, takes afull step up at the initiation of the burst, followed by a ½ step downat about pulse 4, followed by another ½ step down at about pulse 36.

First Preferred Embodiment

[0028] Important features of a first preferred embodiment of the presentinvention are shown in FIG. 4. This embodiment shows a KrF excimer lasersystem useful as a light source for integrated circuit fabrication. Thesystem includes laser chamber 34 which contains a laser gas circulatedwith a motor driven fan (not shown) between two elongated electrodes(not shown) between which electric discharges are produced at rates ofup to 4,000 Hz by a pulse power system 36 controlled by laser controller102. A resonant cavity defined by output coupler 38 and LNP 40 produceslaser beam 42. A portion of laser beam 42 is monitored by very fastwavemeter 104 which provides pulse energy measurements to lasercontroller 102 for feedback control of the pulse power unit 36 formaintaining pulse power within desired limits. Wavemeter also measuresthe center-line wavelength of the laser beam and provides a feedbacksignal to LNP processor which uses the feedback signal to control astepper motor 82 and a PZT driver 80 (shown in FIG. 12A) to adjust theposition of tuning mirror 14 which controls the angle of illumination ofan expanded portion of the laser beam on grating 16 in order to controlthe center-line wavelength of the output beam within desired limits.This preferred embodiment also includes PZT load cell 89 (shown in FIG.4A) which detects high frequency vibrations (caused primarily by the fanand its motor and the discharges) and provides a feedback signal to LNPprocessor 106 which is programmed to provide a corresponding highfrequency control signal to PZT driver 80 to damp down small highfrequency wavelength shifts caused by the high frequency vibration.Important features of this preferred embodiment are described in detailbelow.

Fast Wavemeter Portion of Output Beam Sampled

[0029]FIG. 5 shows the layouts of a preferred wavemeter unit 120, anabsolute wavelength reference calibration unit 190, and a wavemeterprocessor 197. The optical equipment in these units measure pulseenergy, wavelength and bandwidth. These measurements are used withfeedback circuits to maintain pulse energy and wavelength within desiredlimits. The equipment calibrates itself by reference to an atomicreference source on the command from the laser system control processorwhen specified by a laser operator. As shown in FIG. 4 and FIG. 5, thelaser output beam 42 intersects partially reflecting mirror 170, whichpasses about 95.5% of the beam energy as output beam 33 and reflectsabout 4.5% for pulse energy, wavelength and bandwidth measurement.

Pulse Energy

[0030] About 4% of the reflected beam is reflected by mirror 171 toenergy detector 172 which comprises a very fast photo diode 69 which isable to measure the energy of individual pulses occurring at the rate of4,000 to 6,000 pulses per second. The pulse energy is about 5 mJ, andthe output of detector 69 is fed to a computer controller which uses aspecial algorithm to adjust the laser charging voltage to preciselycontrol the pulse energy of future pulses based on stored pulse energydata in order to limit the variation of the energy of individual pulsesand the integrated energy of bursts of pulses.

Linear Photo Diode Array

[0031] The photo sensitive surface of linear photo diode array 180 isdepicted in detail in FIG. 6. The array is an integrated circuit chipcomprising 1024 separate photo diode integrated circuits and anassociated sample and hold readout circuit. The photo diodes are on a 25micrometer pitch for a total length of 25.6 mm (about one inch). Eachphoto diode is 500 micrometer long.

[0032] Photo diode arrays such as this are available from severalsources. A preferred supplier is Hamamatsu. In our preferred embodiment,we use a Model S3903-1024Q which can be read at the rate of up to 4×10⁶pixels/sec on a FIFO basis in which complete 1024 pixel scans can beread at rates of 4,000 Hz or greater. The PDA is designed for 2×10⁶pixel/sec operation but Applicants have found that it can beover-clocked to run much faster, i.e., up to 4×10⁶ pixel/sec. For pulserates greater than 4,000 Hz, Applicants can use the same PDA but only afraction (such as 60% of the pixels are normally read on each scan).

Coarse Wavelength Measurement

[0033] About 4% of the beam which passes through mirror 171 is reflectedby mirror 173 through slit 177 to mirror 174, to mirror 175, back tomirror 174 and onto echelle grating 176. The beam is collimated by lens178 having a focal length of 458.4 mm. Light reflected from grating 176passes back through lens 178, is reflected again from mirrors 174 andthen is reflected from mirror 179 and focused onto the left side of1024-pixel linear photo diode array 180 in the region of pixel 600 topixel 950 as shown in the upper part of FIG. 6A (Pixels 0-599 arereserved for fine wavelength measurement and bandwidth.) The spatialposition of the beam on the photo diode array is a coarse measure of therelative nominal wavelength of the output beam. For example, as shown inFIG. 6A, light in the wavelength range of about 193.350 pm would befocused on pixel 750 and its neighbors.

Calculation of Coarse Wavelength

[0034] The coarse wavelength optics in wavemeter module 120 produces arectangular image of about 0.25 mm×3 mm on the left side of photo diodearray 180. The ten or eleven illuminated photo diodes will generatesignals in proportion to the intensity of the illumination received andthe signals are read and digitized by a processor in wavemetercontroller 197. Using this information and an interpolation algorithmcontroller 197 calculates the center position of the image. Thisposition (measured in pixels) is converted into a coarse wavelengthvalue using two calibration coefficients and assuming a linearrelationship between position and wavelength. These calibrationcoefficients are determined by reference to an atomic wavelengthreference source as described below. For example, the relationshipbetween image position and wavelength might be the following algorithm:

λ=(2.3 pm/pixel)P+191,625 pm

[0035] where P=coarse image central positions.

[0036] Alternatively, additional precision could be added if desired byadding a second order term such as “+( )P².

Fine Wavelength Measurement

[0037] About 95% of the beam which passes through mirror 173 as shown inFIG. 5 is reflected off mirror 182 through lens 183 onto a diffuser atthe input to etalon assembly 184. The beam exiting etalon 184 is focusedby a 458.4 mm focal length lens in the etalon assembly and producesinterference fringes on the middle and right side of linear photo diodearray 180 after being reflected off two mirrors as shown in FIG. 5.

[0038] The spectrometer must measure wavelength and bandwidthsubstantially in real time. Because the laser repetition rate may be4,000 Hz to 6,000 Hz, it is necessary to use algorithms which areaccurate but not computationally intensive in order to achieve thedesired performance with economical and compact processing electronics.Calculational algorithm therefore preferably should use integer asopposed to floating point math, and mathematical operations shouldpreferably be computation efficient (no use of square root, sine, log,etc.).

[0039] The specific details of a preferred algorithm used in thispreferred embodiment will now be described. FIG. 5B is a curve with 5peaks as shown which represents a typical etalon fringe signal asmeasured by linear photo diode array 180. The central peak is drawnlower in height than the others. As different wavelengths of light enterthe etalon, the central peak will rise and fall, sometimes going tozero. This aspect renders the central peak unsuitable for the wavelengthmeasurements. The other peaks will move toward or away from the centralpeak in response to changes in wavelength, so the position of thesepeaks can be used to determine the wavelength, while their widthmeasures the bandwidth of the laser. Two regions, each labeled datawindow, are shown in FIG. 5B. The data windows are located so that thefringe nearest the central peak is normally used for the analysis.However, when the wavelength changes to move the fringe too close to thecentral peak (which will cause distortion and resulting errors), thefirst peak is outside the window, but the second closest peak will beinside the window, and the software causes the processor in controlmodule 197 to use the second peak. Conversely, when the wavelengthshifts to move the current peak outside the data window away from thecentral peak the software will jump to an inner fringe within the datawindow.

[0040] For very fast computation of bandwidth for each pulse atrepetition rates up to the range of 4,000 Hz to 6,000 Hz a preferredembodiment uses the hardware identified in FIG. 7. The hardware includesa microprocessor 400, Model MPC 823 supplied by Motorola with offices inPhoenix, Ariz.; a programmable logic device 402, Model EP 6016QC240supplied by Altera with offices in San Jose, Calif.; an executive anddata memory bank 404; a special very fast RAM 406 for temporary storageof photodiode array data in table form; a third 4×1024 pixel RAM memorybank 408 operating as a memory buffer; and an analog to digitalconverter 410. As explained in U.S. Pat. Nos. 5,025,446 and U.S. Pat.No. 5,978,394, prior art devices were required to analyze a large massof PDA data pixel intensity data representing interference fringesproduced by etalon 184 an photodiode array 180 in order to determinecenter line wavelength and bandwidth. This was a relatively timeconsuming process even with a computer processor because about 400 pixelintensity values had to be analyzed to look for and describe the etalonfringes for each calculation of wavelength and bandwidth. A preferredembodiment of the present invention greatly speeds up this process byproviding a processor for finding the important fringes which operatesin parallel with the processor calculating the wavelength information.

[0041] The basic technique is to use programmable logic device 402 tocontinuously produce a fringe data table from the PDA pixel data as thepixel data are produced. Logic device 402 also identifies which of thesets of fringe data represent fringe data of interest. Then when acalculation of center wavelength and bandwidth are needed,microprocessor merely picks up the data from the identified pixels ofinterest and calculates the needed values of center wavelength andbandwidth. This process reduces the calculation time for microprocessorby about a factor of about 10.

[0042] Specific steps in the process of calculating center wavelengthand bandwidth are as follows:

[0043] 1) With PDA 180 clocked to operate at 2.5 MHz, PDA 180 isdirected by processor 400 to collect data from pixels 1 to 600 at a scanrate of 4,000 Hz and to read pixels 1 to 1028 at a rate of 100 Hz.

[0044] 2) The analog pixel intensity data produced by PDA 180 isconverted from analog intensity values into digital 8 bit values (0 to255) by analog to digital converter 410 and the digital data are storedtemporily in RAM buffer 408 as 8 bit values representing intensity ateach pixel of photodiode array 180.

[0045] 3) Programmable logic device 402 analyzes the data passing out ofRAM buffer 408 continuously on an almost real time basis looking forfringes, stores all the data in RAM memory 406, identifies all fringesfor each pulse, produces a table of fringes for each pulse and storesthe tables in RAM 406, and identifies for further analysis one best setof two fringes for each pulse. The technique used by logic device 402 isas follows:

[0046] A) PLD 402 analyzes each pixel value coming through buffer 408 todetermine if it exceeds an intensity threshold while keeping track ofthe minimum pixel intensity value. If the threshold is exceeded this isan indication that a fringe peak is coming. The PLD identifies the firstpixel above threshold as the “rising edge” pixel number and saves theminimum pixel value of the pixels preceeding the “rising edge” pixel.The intensity value of this pixel is identified as the “minimum” of thefringe.

[0047] B) PLD 402 then monitors subsequent pixel intensity values tosearch for the peak of the fringe. It does this by keeping track of thehighest intensity value until the intensity drops below the thresholdintensity.

[0048] C) When a pixel having a value below threshold is found, the PLDidentifies it as the falling edge pixel number and saves the maximumvalue. The PLD then calculates the “width” of the fringe by subtractingthe rising edge pixel number from the falling edge pixel number.

[0049] D) The four values of rising edge pixel number, maximum fringeintensity, minimum fringe intensity and width of the fringe are storedin the circular table of fringes section of RAM memory bank 406. Datarepresenting up to 15 fringes can be stored for each pulse although mostpulses only produce 2 to 5 fringes in the two windows.

[0050] E) PLD 402 also is programmed to identify with respect to eachpulse the “best” two fringes for each pulse. It does this by identifyingthe last fringe completely within the 0 to 199 window and the firstfringe completely within the 400 to 599 window.

[0051] The total time required after a pulse for (1) the collection ofthe pixel data, and (2) the formation of the circular table of fringesfor the pulse is only about 200 micro seconds. The principal time savingadvantages of this technique is that the search for fringes is occurringas the fringe data is being read out, digitized and stored. Once the twobest fringes are identified for a particular pulse, microprocessor 400secures the raw pixel data in the region of the two fringes from RAMmemory bank 406 and calculates from that data the bandwidth and centerwavelength. The calculation is as follows:

[0052] Typical shape of the etalon fringes are shown in FIG. 18B. Basedon the prior work of PLD 402 the fringe having a maximum at about pixel180 and the fringe having a maximum at about pixel 450 will beidentified to microprocessor 400. The pixel data surrounding these twomaxima are analyzed by microprocessor 400 to define the shape andlocation of the fringe. This is done as follows:

[0053] A) A half maximum value is determined by subtracting the fringeminimum from the fringe maximum dividing the difference by 2 and addingthe result to the fringe minimum. For each rising edge and each fallingedge of the two fringes the two pixels having values of closest aboveand closest below the half maximum value. Microprocessor thenextrapolates between the two pixel values in each case to define the endpoints of D1 and D2 as shown in FIG. 18B with a precision of {fraction(1/32)} pixel. From these values the inner diameter D1 and the outerdiameter D2 of the circular fringe are determined.

Fine Wavelength Calculation

[0054] The fine wavelength calculation is made using the coursewavelength measured value and the measured values of D1 and D2.

[0055] The basic equation for wavelength is:

λ=(2*n*d/m)cos(R/f)  (1)

[0056] where

[0057] λ is the wavelength, in picometers,

[0058] n is the internal index of refraction of the etalon, about1.0003,

[0059] d is the etalon spacing, about 1542 um for KrF lasers and about934 μm for ArF lasers, controlled to +/−1 um,

[0060] m is the order, the integral number of wavelengths at the fringepeak, about 12440,

[0061] R is the fringe radius, 130 to 280 PDA pixels, a pixel being 25microns,

[0062] f is the focal distance from the lens to the PDA plane.

[0063] Expanding the cos term and discarding high order terms that arenegligibly small yields:

λ=(2*n*d/m)[1−(½)(R/f)²]  (2)

[0064] Restating the equation in terms of diameter D=2*R yields:

λ=(2*n*d/m)[1−(⅛)(D/f)²]  (3)

[0065] The wavemeter's principal task is to calculate λ from D. Thisrequires knowing f, n, d and m. Since n and d are both intrinsic to theetalon we combine them into a single calibration constant named ND. Weconsider f to be another calibration constant named FD with units ofpixels to match the units of D for a pure ratio. The integer order mvaries depending on the wavelength and which fringe pair we choose. m isdetermined using the coarse fringe wavelength, which is sufficientlyaccurate for the purpose.

[0066] A couple of nice things about these equations is that all the bignumbers are positive values. The WCM's microcontroller is capable ofcalculating this while maintaining nearly 32 bits of precision. We referto the bracketed terms as FRAC.

FRAC=[−(⅛)(D/FD)²]  (4)

[0067] Internally FRAC is represented as an unsigned 32 bit value withits radix point to the left of the most significant bit. FRAC is alwaysjust slightly less than one, so we get maximal precision there. FRACranges from [1−120E-6] to [1−25E-6] for D range of {560˜260} pixels.

[0068] When the ND calibration is entered, the wavemeter calculates aninternal unsigned 64 bit value named 2ND=2*ND with internal wavelengthunits of femtometers (fm)=^ 10−15 meter=0.00 pm. Internally we representthe wavelength λ as FWL for the fine wavelength, also in fin units.Restating the equation in terms of these variables:

FWL=FRAC*2ND/m  (5)

[0069] The arithmetic handles the radix point shift in FRAC yielding FWLin fm. We solve for m by shuffling the equation and plugging in theknown coarse wavelength named CWL, also in fin units:

m=nearest integer(FRAC*2ND/CWL)  (6)

[0070] Taking the nearest integer is equivalent to adding or subtractingFSRs in the old scheme until the nearest fine wavelength to the coarsewavelength was reached. Calculate wavelength by solving equation (4)then equation (6) then equation (5). We calculate WL separately for theinner and outer diameters. The average is the line center wavelength,the difference is the linewidth.

Bandwidth Calculation

[0071] The bandwidth of the laser is computed as (λ₂−λ₁)/2. A fixedcorrection factor is applied to account for the intrinsic width of theetalon peak adding to the true laser bandwidth. Mathematically, adeconvolution algorithm is the formalism for removing the etalonintrinsic width from the measured width, but this would be far toocomputation-intensive, so a fixed correction Δλε is subtracted, whichprovides sufficient accuracy. Therefore, the bandwidth is:${\Delta \quad \lambda} = {{( \frac{D_{2} - D_{1}}{2} ) - {\Delta\lambda}} \in}$

[0072] Δλε depends on both the etalon specifications and the true laserbandwidth. It typically lies in the range of 0.1-1 pm for theapplication described here.

Feedback Control of Pulse Energy

[0073] Based on the measurement of pulse energy of each pulse asdescribed above, the pulse energy of subsequent pulses are controlled tomaintain desired pulse energies and also desired total integrated doseof a specified number of pulses all as described in U.S. Pat. No.6,005,879, Pulse Energy Control for Excimer Laser which is incorporatedby reference herein.

Fast Feedback Control of Wavelength

[0074] Wavelength of the laser may be controlled in a feedbackarrangement using measured values of wavelengths and techniques known inthe prior art such as those techniques described in U.S. Pat. No.5,978,394, Wavelength System for an Excimer Laser also incorporatedherein by reference. Applicants have recently developed techniques forwavelength tuning which utilize a piezoelectric driver to provideextremely fast movement of tuning mirror. Some of these techniques aredescribed in U.S. patent application Ser. No. 608,543, Bandwidth ControlTechnique for a Laser, filed Jun. 30, 2000 which is incorporated hereinby reference. FIGS. 22A and 22B are extracted from that application andshow the principal elements of this technique. A piezoelectric stack isused for very fast mirror adjustment and larger slower adjustments areprovided by a prior art stepper motor operating a lever arm. Thepiezoelectric stack adjusts the position of the fulcrum of the leverarm.

Fast Mirror Adjustment

[0075]FIGS. 4, 4A and 4B show an arrangement permitting very fastadjustment of mirror 14. This embodiment is a major speed up as comparedto the stepper motor drive system described above but not quite fastenough for pulse-to-pulse adjustment. As indicated above, earliermethods of mirror positioning required about 7 ms to move mirror 14,making pulse-to-pulse wavelength correction at 2000 Hz out of thequestion. In that earlier technique, a lever arm pivoted about a pivotaxis to produce a 1 to 26.5 reduction in the mirror movement compared tothe stepper position movement. The prior art stepper has a total travelof ½ inch (12.7 mm) and 6000 steps so that each step is a distance ofabout 2 microns. With the 1-26.5 reduction, one step moves the mirrorabout 75 nm which typically changes the wavelength of the laserwavelength about 0.1 pm. In the fast acting technique shown in FIG. 4A,a piezo stack 80 has been added at the pivot position of the lever arm.A preferred piezo stack is Model P-840.10 supplied by Physik InstrumenteGmbH with offices in Waldbronn, Germany.

[0076]FIG. 8 is a drawing showing detail features of this preferredembodiment of the present invention. Large changes in the position ofmirror 14 are produced by stepper motor through a 26.5 to 1 lever arm84. In this case a diamond pad at the end of piezoelectric drive 80 isprovided to a contact spherical tooling ball at the fulcrum of lever arm84. The contact between the top of lever arm 84 and mirror mount 86 isprovided with a dow pin on the lever arm and four spherical ballbearings mounted (portions of only two of which are shown) on the mirrormount as shown at 85. Piezoelectric drive 80 is mounted on the LNP framewith piezoelectric mount 80A and the stepper motor is mounted to theframe with stepper motor mount 82A. Mirror 14 is mounted in mirror mount86 with a three point mount using three aluminum spheres, only one ofwhich are shown in FIG. 8. Three springs 14A apply the compressive forceto hold the mirror against the spheres. This embodiment includes abellows 87 with drum flexure 88 to isolate the piezoelectric drive fromthe environment inside the LNP. This isolation prevents UV damage to thepiezoelectric element and avoid possible contamination caused by outgassing from the piezoelectric materials. This embodiment also includesload cell 89 mounted near the top of PZT stack 80 to provide a feedbackhigh frequency vibration signal as discussed in more detail below.

[0077] This stack will produce linear adjustment of about 3.0 micronswith a drive voltage change of 20 volts. This range is equivalent toabout ±20 steps of the stepper motor.

[0078] The stack responds to a control signal within less than 0.1milliseconds and the system can easily respond to updated signals at afrequency of 2000 Hz. Preferably the wavemeter will be fast enough toprovide a feedback signal prior to each pulse based on data from theprior pulse. The feedback data could be based not on the previous pulsebut the pulse prior to the previous pulse to allow plenty of time forthe wavelength calculation. Even every other pulse would provide afactor of 7 improvement over the prior art design with a 7 millisecondlatency. Therefore, much faster feedback control can be provided. Onepreferred feedback control algorithm is described in FIG. 9. In thisalgorithm the wavelength is measured for each pulse and an averagewavelength for the last four and last two pulses is calculated. Ifeither of the average deviate from the target wavelength by less than0.02 pm, no adjustment is made. If both deviate more than 0.02 pm fromthe target, an adjustment is made to the mirror assembly bypiezoelectric stack 80 to provide a wavelength correction. Which of thetwo averages is used is determined by how much time had elapsed sincethe last adjustment. The piezoelectric stack is maintained within itscontrol range by stepping the stepper motor as the stack approaches 30and 70 percent of its range (alternatively other ranges such as 45 and55 percent could be used instead of the 30 and 70 percent range values).Since the stepper motor requires about 7 ms to complete a step, thealgorithm may make several piezo adjustments during a stepper motorstep. Alternatively, instead of the FIG. 9 algorithm a traditionalproportional algorithm as described for the L region in FIG. 11.

Vibration Control

[0079] This preferred embodiment provides feedback control ofcenter-line wavelength shifts due to low amplitude, high frequencyvibration-type disturbances of laser equipment (especially the opticalcomponents) due primarily to the rotating fan and its motor driver andto the electric discharges. In this preferred embodiment, piezoelectricload cell 89 (approximately 0.25 inch diameter×0.02 inch) is locatednear the top of PZT stack 80 as shown in FIG. 8. The load cellpreferably is packaged as described in U.S. Pat. No. 5,656,882 (herebyincorporated by reference herein). The load cell is located between aspherical contact 90 and drum feature 88.

[0080] In this embodiment, electrical connection to the two major sidesof the piezoceramic sensor is achieved through insulated copper traces.Alternatively, a bare piece of piezoceramic material could be utilizedas the load cell along with some means of electrical connection (e.g. 30AWG copper wire) to the electrically conductive surfaces of thepiezoceramic material. The load sensor is utilized to measure the forcesapplied to the R_(max) holder through the spherical contacts, the drumflexure, and the StepperMike lever by the piezoceramic stack. Theseforces are the result of the vibration-type disturbances referred toabove.

[0081] PZT load cell 89 produces 10's of volts in response to forcesapplied by the PZT stack 80 or in response to vibrations in the Rmaxholder 84 or connecting structures. The signal levels produced by thePZT load cell 89 are generally proportional to the forces or vibrations.The sensor can easily measure forces or vibrations at a frequency inexcess of approximately 100 kHz. The sensor can detect forces as smallas 0.01 Newton.

Active Damping

[0082] In the FIG. 8 embodiment the load cell 89 is used in conjunctionwith an active damping module 320 and the PZT stack 80 to reducestructural vibrations to improve line center stability. Morespecifically, active damping is applied to reduce the center wavelengtherror (typically computed based on wavelength data collected from a 30pulse “window” of data, the average of which is called a “30 pulsemoving average”) and a 30 pulse moving standard deviations arecalculated. In FIG. 8, the load sensor 89 emits an electrical charge 330that is proportional to the applied load or vibration. This charge isfed into a charge amplifier circuit (with signal conditioner) 332 usedto condition the signal by converting it in to a voltage proportional tothe charge. The details of the design of such charge amplifier circuitscan be found in a PCB Piezotronics Inc. technical support documententitled “Introduction to Signal Conditioning for ICP and ChargePiezoelectric Sensors” or similar instruction manuals dealing withvibration sensor design and implementation.

[0083] Voltage signal 331 is fed into processor 333 which in this caseis Model SBC67 supplied by Innovative Integration Inc. with offices inSimi Valley, Calif. This processor is a high performance stand-alonedigital signal processor single board computer featuring analog inputand output capability. The voltage signal 331 is fed into an analoginput. This analog input is then converted to a digital signal that isused by the processor with an appropriate algorithm (described below) tocreate a digital output signal which is then converted to an analogoutput signal 334. This output signal 334 is then summed (electrically)with the center wavelength control signal coming from the LNP processor106 and applied as a command signal to the PZT stack.

[0084] The filter (feedback control algorithm) may be designed using thestandard Linear Quadratic Regulator approach, ensuring that thepiezoelectric stack actuator control voltages do not exceed the stackactuator device or amplifier limits. Actuator voltages in the closedfeedback loop are proportional to the load sensor signal 89 associatedwith vibration in the structure. Several types of control filter areapplicable to active damping architectures. In the present case, with asingle actuator/sensor pair and a few well-spaced (in frequency) modesto be controlled, narrowband filters provide the correct combination ofgain and phase at the natural frequency of interest, while adding littleor no feedback gain elsewhere, enhancing stability. Such a controlapproach has been called Positive Position Feedback. The controlapproaches are discussed at length by Fanson, J. L., and Caughey, T. K.,“Positive Position Feedback Control for Large Space Structures,” AIAAJournal, Vol. 28, ,pp. 717-724, 1990. These approaches also apply to theadjoint problem of force feedback to a position actuator. Control designshould be tailored for the specific application. A preferred techniqueis to create a state-space plant model from transfer function data usinga software package such as the Smart ID system identification softwarepackage commercially available from Active Control Experts, Inc. withoffices in Cambridge, Mass. The filter (or controller) preferably isdesigned using computer simulation and techniques such as thoseexplained by Fanson (see above) and those provided in The ControlHandbook, William S. Levine, Editor, CRC Press 1996.

[0085] Applicants have conducted studies to determine the source ofthese disturbances and have determined that the fast random component ofthe wavelength shift is caused by persistent disturbances. Most of therandom component is caused by the blower. This disturbance is fairlybroadband and random. It excites numerous resonances up to 1500 Hz, plusone significant outlier at a much higher frequency. Applicants' testshave shown that the frequencies excited do not shift significantly as afunction of blower speed. However, the magnitudes of the disturbances atvarious frequencies are affected by blower speed.

[0086] Modal identification of the laser system was used in conjunctionwith experimental data to determine what components of the system werevibrating. Modal identification involved the mapping of displacements ofdifferent portions of the laser system to determine the general shape ofthe vibration at a given frequency. Accelerometers, strain gages, PZTstrain sensors, and a load cell were used to construct the modalresponse of the system at various frequencies of interest. The modalresponse of the system was determined for certain frequencies that weredetected (such as 44 Hz, 178 Hz, 590 Hz and 900-1000 Hz). Applicantsdetermined that for the particular laser tested, the 44 Hz moderepresented the cantilevered vibration of the LNP and that the 178 Hzvibration was due to the LNP bending and twisting. The 590 Hz moderepresents complex motion of the LNP in which opposing portions of theLNP structure vibrate in opposition (breathing motion). This mode is thealiased frequency of a structural mode at 2682 Hz. The cluster of modesin the 900-1000 Hz range represents a set of modes that are aliased downfrom a cluster of structural modes located between 1100-1200 Hz. Astructural mode located in this cluster is a local mode of the steeringmirror. The location of this group of modes may vary as a function ofthe specific design of the mirror assembly.

[0087] Applicants determined that about 50% of the center wavelengthvariance was due to a set of structural resonances between roughly 1100Hz and 1200 Hz which were randomly excited by the blower in the chamber.In this embodiment, these modes are actively controlled, in theaggregate, by the PZT stack, through feedback of the force measured byload cell 89 in the PZT stack drivetrain operated at a high sample rateof about 20 kHz. This sampling rate is about 20 times faster than thefeedback signal available from the wavemeter described above. Forpurposes of active feedback it is usually desirable to have a feedbacksensor signal that can be sampled at a rate of at least approximately10× higher than the frequency of the mode that you would like tocontrol. Ideally, a higher bandwidth measurement signal that iscorrelated to the center wavelength of error would be available as thefeedback measurement. At this point, the PZT load cell signal is usedfor feedback.

[0088] The PZT load cell is very stiff and it is sensitive enough tomeet requirements imposed by the small forces in the system, andprovides a high-bandwidth “always on” signal for active dampingfeedback. The “always on” feature also eliminates any problem withtransient controller dynamics at the start of a burst unlike thedisturbance rejection control which is accomplished through feedback oflight measurements.

[0089] As explained above, the feedback control solution implementedconsists of the load cell sensor 89 and PZT stack actuator as shown inFIG. 8. This high frequency feedback control system preferably includespre- and post-amplifiers, designed to obtain sufficiently cleanfeedback/feedforward signals and designed to convert the control signalsynthesized by the processor into the correct actuator signal usingtechniques as explained in U.S. Pat. No. 5,656,882. The actuators impartforces on the mirror, which work to oppose or counter the disturbancesresulting in reduced vibrations caused wavelength shifts.

[0090] Actual tests performed by Applicants demonstrated that thisfeedback vibration control reduced the wavelength error autospectrum byabout 20% from 0.037 pm RMS to about 0.029 pm RMS. In addition, actualtests performed by Applicants demonstrated that this feedback vibrationcontrol reduced a moving-window standard deviation by about 33% from0.048 pm to 0.030 pm.

[0091]FIG. 12 and FIGS. 13A and 13B show actual test data from a laserfitted with the FIG. 8 embodiment in which active damping vibrationcontrol is implemented. The graph in FIG. 12 is a plot of the wavelengtherror autospectrum (without the pm/V scale factor included) acquired ata 2100 repetition rate with the active damping control loop open andclosed. Graphs 13A1, 2 and 3 and 13B1, 2 and 3 are plots of amoving-window data computed from a number of bursts at 2100 Hzrepetition rate. These graphs show line center error, mean wavelengthdeviation from target and mean standard deviation. The 13A plots areopen loop and the 13B plots are closed loop. The charts show asubstantial reduction in the standard deviation with the closed loopconfiguration.

Adaptive Feedforward

[0092] Preferred embodiments of the present invention includesfeedforward algorithms. These algorithms can be coded by the laseroperator based on known burst operation patterns such as those shown inFIG. 1 and FIGS. 2A-D. Alternatively, this algorithm can be adaptive sothat the laser control detects burst patterns such as those shown in theabove charts and then revises the control parameters to provideadjustment of mirror 14 in anticipation of wavelength shifts in order toprevent or minimize the shifts.

[0093] The adaptive feedforward technique involves building a model ofthe chirp at a given rep rate in software, from data from one or moreprevious bursts and using the PZT stack to invert the effect of thechirp.

[0094] To properly design the chirp inversion, two pieces of informationare needed: (1) the pulse response of the PZT stack, and (2) the shapeof the chirp. For each repetition rate, deconvolution of the chirpwaveform by the pulse response of the PZT stack will yield a sequence ofpulses, which, when applied to the PZT stack (with appropriate sign),will cancel the chirp. This computation can be done off line through asurvey of behavior at a set of repetition rates. The data sequencescould be saved to tables indexed by pulse number and repetition rate.This table could be referred to during operation to pick the appropriatewaveform data to be used in adaptive feedforward inversion. It is alsopossible, and in fact may be preferable, to obtain the chirp shape modelin almost real-time using a few bursts of data at the start of operationeach time the repetition rate is changed. The chirp shape model, andpossibly the PZT pulse response model as well, could then be updated(e.g. adapted) every N-bursts based on accumulated measured errorbetween model and data. A preferred algorithm is described in FIG. 11.

[0095] The chirp at the beginning of bursts of pulses can be controlledusing the algorithm described in FIG. 11. The letter k refers to thepulse number in a burst. The burst is separated into two regions, a kregion and an l region. The k region is for pulse numbers less thank_(th) (defining a time period long enough to encompass the chirp).Separate proportional constant P_(k), integral constant I_(k) andintegral sum of the line center error ELCEk are used for each pulsenumber. The PZT voltage for the corresponding pulse number in the kregion in the next burst is determined by these constants and sums.After the kth pulse, a traditional proportional integral routinecontrols the PZT voltage. The voltage for next pulse in the burst willbe the current voltage plus P*LCE+I*ΣLCE. A flow diagram explaining themajor steps in this algorithm is provided in FIG. 11.

Test Results

[0096]FIG. 9 shows actual test data from a laser fitted with the FIG. 8embodiment. The graph is a plot of the deviation from target wavelengthof the average of 30 pulse windows. The deviation is reduced from about0.05 pm to about 0.005 pm.

[0097] Many other embodiments of the present invention may be developedfor accomplishing the same or similar goals in the laser system oranother piece of capital equipment or subsystem for the semiconductor,electronics, optical, or medical markets.

[0098] For example, other sensors may be used for control. Sensorsconsidered would be suitably or sufficiently correlated with theperformance metric (in the case of stability of the laser wavelength,the wavelength and it's standard deviation are exemplary performancemetrics). In addition, sensors would also need to have adequateproperties, including, resolution, accuracy, sensitivity, and signalbandwidth. Other sensors considered include strain sensors for measuringstrain in a structure or component of the laser. Various positionsensors also would be considered including capacitive, inductive, andoptical displacement sensors. Acceleration sensors are also consideredwhere appropriate. In addition, a sensor capable of faster measurementof the light signal could be of use in reducing vibration and therebyimproving wavelength stability.

[0099] Other actuators are also considered for replacing or augmentingthe stepper motor or the PZT stack. These include electromagnetic andinductive motors and rotary piezoceramic motors.

[0100] In addition, alternate processors or active damping modulessolutions are considered. For example, all processing could be performedusing only one system suitably designed to deal with all the signalconditioning, power electronics, and controller processing, controllerimplementation, data logging, and other needs.

[0101] Therefore, the appended claims are to encompass within theirscope all such changes and modifications as fall within the true spiritand scope of this invention.

We claim:
 1. An electric discharge laser with precision wavelengthcontrol for controlling center wavelength of laser beams produced bysaid laser said laser comprising: A) a laser chamber, B) an elongatedelectrode structure enclosed within said chamber comprising an elongatedanode and an elongated cathode separated by a distance defining adischarge region, said discharge region defining a long dimension in abeam direction, C) a laser gas contained in said chamber, D) a fan forcirculating said laser within said chamber and through said dischargeregion, and E) a precision wavelength control system comprising: 1) awavemeter for measuring said center wavelength; 2) a wavelengthselection unit comprising: a) a beam expander configured to expand aportion of a laser beam produced in said chamber in order to produce anexpanded beam, b) a grating, c) an illumination angle control unit foradjusting illumination angles of said expanded beam on said grating saidcontrol unit comprising: i) a piezoelectric driver; ii) at least onefeedback control system configured to control said piezoelectric driver.2. A laser as in claim 1 wherein said illumination angle control unitcomprises a tuning mirror and said piezoelectric driver is configured tocontrol positions of said mirror.
 3. A laser as in claim 2 wherein saidlaser produces an early occurring chirp with a duration of a fewmilliseconds.
 4. A laser as in claim 2 wherein said illumination anglecontrol unit comprises a stepper motor.
 5. A laser as in claim 2 whereinsaid illumination angle control unit comprises a processor programmedwith a learning algorithm for learning the shape of the early occurringchirp.
 6. A laser as in claim 1 wherein said illumination angle controlunit is configured to provide mirror adjustments in time periods of lessthan 2 milliseconds.
 7. A laser as in claim 1 wherein said illuminationangle control unit is configured to provide mirror adjustments in timeperiods of less than 500 microseconds.
 8. A laser as in claim 7 whereinsaid illumination angle control unit also comprises a stepper motorhaving an external spindle.
 9. A laser as in claim 8 wherein saidillumination angle control unit also comprises a lever arm pivoted abouta pivot axis to provide a de-magnification of linear movements of saidexternal spindle.
 10. A laser as in claim 1 wherein said active chirpmitigation means comprises a stepper motor for coarse wavelength controland a piezoelectric device for fine wavelength control .
 11. A laser asin claim 1 wherein said illumination angle control unit also comprises aload cell positioned to measure vibration at least one place within saidillumination angle control unit.
 12. A laser as in claim 11 wherein saidat least one place is a tuning mirror.
 13. A laser as in claim 12wherein said illumination angle control unit is configured to controlpositions of said tuning mirror using said piezoelectric driver based onsignals from said load cell.
 14. A laser as in claim 1 wherein saidprecision wavelength control system also comprises a processorprogrammed with an computer program for controlling said illuminationangle control unit during bursts of laser pulses based on historicalpulse data from previous bursts of pulses.
 15. A laser as in claim 14wherein said computer program comprises a learning algorithm permittingsaid program to learn needed adjustments of said tuning mirror toproduce laser beams having wavelengths within a desired range.
 16. Anelectric discharge laser with precision wavelength control forcontrolling center wavelengths of laser beams produced by said lasersaid laser comprising: A) a laser chamber, B) an elongated electrodestructure enclosed within said chamber comprising an elongated anode andan elongated cathode separated by a distance defining a dischargeregion, said discharge region defining a long dimension in a beamdirection, C) a laser gas contained in said chamber, D) a fan forcirculating said laser within said chamber and through said dischargeregion, E) a wavemeter for measuring the centerline wavelength, F) awavelength tuning mechanism including at least one piezoelectric drive,G) a feedback control system for controlling said tuning mechanism usingmeasurement information from said wavemeter in order to actively controlwavelength chirp.
 17. A laser as in claim 16 wherein said tuningmechanism comprises a tuning mirror and an adjusting mechanism foradjusting the position of the tuning mirror in advance of a burst ofpulses to mitigate a chirp occurring in an early part of the burst. 18.A laser as in claim 16 wherein the early occurring chirp has a durationof less than one millisecond.
 19. A laser as in claim 16 wherein saidadjusting mechanism comprises a stepper motor.
 20. A laser as in claim16 wherein said adjusting mechanism comprises a processor programmedwith a learning algorithm for learning the shape of the early occupyingchirp.
 21. A laser as in claim 16 wherein said tuning mechanism alsocomprises a stepper motor having an external spindle.
 22. A laser as inclaim 21 wherein said tuning mechanism also comprises a lever armpivoted about a pivot axis to provide a de-magnification of linearmovements of said external spindle.
 23. A laser as in claim 16 whereinsaid tuning mechanism comprises a stepper motor for coarse tuning and apiezoelectric device for fine tuning.
 24. An electric discharge laserwith precision wavelength control for controlling center wavelengths oflaser beams produced by said laser said laser comprising: A) a laserchamber, B) an elongated electrode structure enclosed within saidchamber comprising an elongated anode and an elongated cathode separatedby a distance defining a discharge region, said discharge regiondefining a long dimension in a beam direction, C) a laser gas containedin said chamber, D) a fan for circulating said laser within said chamberand through said discharge region, E) a wavemeter for measuring thecenterline wavelength, F) a wavelength tuning mechanism comprising atuning mirror and a piezoelectric driver for driving said tuning mirror,G) a feedback control system for controlling said tuning mechanism usingmeasurement information from said wavemeter in order to actively controlwavelength chirp, H) a load cell configured to measure vibrations ofsaid tuning mirror, I) a feedback control system for controlling saidtuning mechanism based on signals from said load cell.
 25. A vibrationcontrol system for a line narrowed laser having a wavelength selectinggrating said system comprising: A) an illumination angle control unitfor controlling illumination angles on said grating unit, B) apiezoelectric driver unit for driving said control unit, C) apiezoelectric load cell monitoring unit for monitoring forces on saidcontrol unit and providing a feedback signal to said piezoelectricdriver unit.
 26. A system as in claim 25 wherein said illumination anglecontrol unit comprises a pivotable mirror.
 27. A system as in claim 25wherein said illumination angle control unit comprises a pivotablegrating.